This invention relates to a steel sheet having a protective coating and production thereof. More particularly, this invention relates to a steel sheet having a thermoplastic acrylic coating that is impervious to moisture, oil, dirt, and the like during handling and forming, is capable of being welded and adhesively bonded, and has sufficient lubricity for deep drawing the sheet without needing additional lubricant.
Forming of steel sheets requires a lubricant to prevent scoring and galling during stamping. Liquid lubricants such as oil generally are considered unsatisfactory for a number of reasons. Oils have a tendency to age over time, especially when exposed to elevated temperature such as stamping press temperatures and storage temperatures. Aging causes oils to polymerize and become difficult to remove after forming a sheet. This especially is apparent on relatively porous zinc electroplated steels. Oil may become embedded in the sheet surface and causes an inconsistent appearance when the sheet is given a phosphate coating after cleaning. Oil also may remain trapped inside hem flange joints during stamping. If the sheet is then painted, the oil volatilizes during curing of the paint leading to cratering. Cratering of the paint results in an unacceptable surface appearance and poor corrosion. Oils also are a safety concern in the work area, result in a severe housekeeping problem, and generally cause operator dissatisfaction because of odor and air contamination.
In the automotive industry, it is known to apply a dry coating containing a soap to steel sheet. Dry soap coatings have very good lubricating characteristics for press forming and have minimized the safety concern since additional external liquid lubricant generally is not required in forming operations. Unfortunately, soaps are hydrophilic in nature and moisture causes tackiness in the soap film. This moisture may result in stamping die pickup of the soap, short blanking of the sheet, and sticking of a formed article in the dies. These problems cause dings, dents, and the like when removing formed articles from the dies. The moisture also may result in poor corrosion protection.
U.S. Pat. No. 4,411,145 discloses a composition for an aqueous solution containing an acrylic polymer, a wax, and a finely divided molybdenum disulfide for coating steel for can making. The patent suggests a dry coating formed from the solution does not rub off during handling or ironing of the sheet and has sufficient lubricity to prevent scoring and galling during formation of cans. The aqueous solution is applied as a liquid film to one side of the sheet by spraying, using a roller or wiping and thereafter dried. After the sheet is ironed, the dry film is removed from the cans by washing in an alkaline solution. The patent discloses a dry coating thickness of 5-200 mg/ft.sup.2 molybdenum disulfide.
Unlike thin gauge steel for can making, heavier gauge steels for deep drawing applications such as automobile and appliances have more severe surface requirements. The sheet surface generally is rougher, particularly for galvanized sheet, and the types of dry coatings described above are inadequate because the coating does not form a continuous, polymerized film impervious to contamination. Furthermore, hold down pressure for deeply drawn sheets may be inadequate to form articles having consistent dimensions and uniform stretching/stiffness characteristics. Formed articles also may require welding or adhesive bonding prior to removal of the dry coating. The coating not only must have uniform thickness to provide the necessary lubricity but also not insulate sheets or formed articles during resistance welding. The coating must still be impervious to contamination after forming to protect the sheet base metal from corrosion while the unfinished formed articles are in storage awaiting further processing. For formed articles to be painted, the lubricant film must be able to be completely removed so that the painting characteristics are enhanced.
Accordingly, there remains a long felt need for a dry organic coating for deeply drawn steel sheet that is tack free, is hydrophobic, provides good corrosion protection, and has sufficient lubricity so that additional lubricant is not required during forming of the sheet. The coating also must be impervious to contamination during handling of the sheet before and after forming and during welding and easily be removed after forming so that the article surface has enhanced painting characteristics.